It's in there, in between the articles about the second amendment and how the government

will be coming after you.

I thought I’d go ahead and explain in a post about the
making of the life restoration of Pelagiarctos, and discuss what it may
have looked like in life. As detailed in our phylogenetic analysis, Pelagiarctos
is most closely related to the late Miocene walrus Imagotaria downsi.
Imagotaria downsi is known from the Santa Margarita Sandstone and
Sisquoc Formation of Northern and southern California
(respectively), and is early late Miocene in age (Tortonian stage – 9-12 Ma).
The type specimen of Imagotaria downsi is sort of a cruddy specimen, but a
beautiful collection of well preserved fossils was reported by Charles
Repenning and Richard Tedford in 1977 from the Santa Margarita Sandstone near Santa
Cruz, California. Two skulls
including an adult female (“Rep’s Girl” as some marine mammal paleontologists
call it) and a subadult male are sea lion like in their morphology. They have
long snouts, the skulls are flat-topped, and have low saggittal crests, large
canines, and deep mandibles. Although superficially sea lion-like in general
form, they lack the supraorbital shelves typical of otariids.

The female skull of Imagotaria downsi from Santa Cruz, affectionately known as "Rep's girl".

The lack of similarity between Pelagiarctos and the
modern walrus is evident in its morphology. In the modern walrus, the canines
are reduced in size to small pegs, no larger than the premolars (and are thus
called ‘premolariform’); the incisors are totally absent, and the “chin” of the
modern walrus mandible tapers to a triangular point that lacks teeth and
instead has a longitudinal furrow. The transverse tapering of the mandible
accommodes the hugely expanded tusks, which for the uninitiated, are the upper
canines. The reduction in size of the lower canines presumably also permits
enlarged upper tusks – while the complete loss of incisors probably allows an
unobstructed pathway for suction into the oral cavity (see the section on odobenid
dental and mandibular evolution in Odobenidae in our paper).

Rough line drawing of "Rep's girl".

Instead, Pelagiarctos – like Imagotaria –
appears to be rather sea lion-like in overall morphology, perhaps something
similar to one of the larger, more robust sea lions such as the New Zealand sea
lion (Phocarctos hookeri), South American sea lion (Otaria byronia),
or Steller’s sea lion (Eumetopias jubatus); the mandible of California
sea lions is noticeably smaller and less ‘deep’ than Pelagiarctos. So,
we have an overall idea of the shape of the head of Pelagiarctos. To
start, I took a line drawing of the skull and mandible of Imagotaria downsi,
and in adobe illustrator reduced them to the same mandible size. Then, I took
the facial part of the skull, and shortened it to fit the short toothrow of Pelagiarctos.
Figuring that the rostrum of Pelagiarctos would have probably been
deeper and more robust like the mandible, I also made the facial region more
dorsoventrally deep.

Cranial reconstruction of Pelagiarctos sp. based on the proportions of Imagotaria downsi.

Then, using this new skull reconstruction, I sketched it in
an oblique view by using a reference photo of an Imagotaria skull in the
same oblique view, while adding in the changes in proportions. I made sure to
sketch it in an angle where I had a photograph of the new Pelagiarctos specimen,
so that I could later on digitally overlay the photo of the mandible.

What did Pelagiarctos look like? More like a sea lion, or a fellow walrus? Due to its early position and early timing in pinniped evolution, it probably had some sort of external ears. It was not a gigantic cold-adapted pinniped like the walrus, so it may have primitively retained abundant fur or hair. It's long whiskers as I've reconstructed it are consistent with a piscivorous habit, rather than the molluskivorous feeding behavior of the modern walrus. My initial black and white graphite drawing is shown here.

So, we’ve got a general shape of the skull and head – but
what would it look like? There are still a couple more considerations. For
example – would it have had fur? Long vibrissae (whiskers), or short vibrissae?
Would it have had thick blubber instead, like the modern walrus? And what about
ears – modern walruses (and true seals) don’t have external ears, but sea lions
and fur seals (Otariidae) have dinky little ear flaps. Before I continue with
this discussion, I must stress that this is all highly speculative. Using the
“extant phylogenetic bracket”, and assuming that molecular analyses have
correctly identified sea lions and walruses as sister taxa, we can infer that
it would look closest to a sea lion or a modern walrus. Okay, that basically
includes all of the aforementioned features. The modern walrus is technically
closer – but it is a highly derived animal, whereas Pelagiarctos is a
very generalized sea lion-like pinniped. I reconstructed Pelagiarctos with
external ear flaps to reflect the fact that most early pinnipeds probably had
ear flaps or even large external ears (e.g. like an otter). After all, external
ears are primitive, and it would be silly to assume that true seals and
walruses have lacked ears throughout their evolutionary history. Considering
the molecular support for a sea lion + walrus clade, it appears that external
ear loss is convergent in the walrus and true seals anyway. What about fur,
then? Only a few pinnipeds truly lack dense fur or hair – the walrus, and the
elephant seals. The southern elephant seal and the walrus are both high latitude, cold water adapted - but they are also substantially larger than Pelagiarctos. Given the temperate latitude and similarity in size of
Pelagiarctos with modern sea lions, which lack fur but have dense hair – it can be parsimoniously reconstructed as
“fuzzy”. On that note, I really ought to talk about Heather Liwanag's awesome study on the evolution of marine carnivore fur/hair. Lastly, I reconstructed it with long whiskers because it’s a pelagic
hunter – the short, stubby whiskers of the modern walrus are an adaptation for
“feeling” benthic invertebrates and sediment.

And the final reconstruction, all colorized and everything. I had a lot of fun doing this reconstruction, and it seemed to do the trick.

Anyway, this concludes my series of posts on the new study by Morgan and I. Hope you enjoyed reading it (and hopefully the actual paper as well).

References-

Boessenecker, R.W. and M. Churchill. 2013. A reevaluation of the morphology, paleoecology, and phylogenetic relationships of the enigmatic walrus Pelagiarctos. PLoS One 8(1) e54311. doi:10.1371/journal.pone.0054311.

Sunday, February 10, 2013

I walked into my labmate Cheng-Hsiu Tsai's office recently and saw that he had a new desktop background - there were some specimens I didn't even recognize. It's a collage he made with nearly every published fossil mysticete he could find (and some modern mysticetes). Tsai, who is currently on a two month research trip to Taiwan, Japan, and Australia - said it would be OK for me to share it here with you all - so, enjoy!

Thursday, February 7, 2013

Sorry for the short break – I’ve been fairly busy the last
two weeks doing a number of things, but most of all revising a manuscript that
was accepted last week on barnacle-encrusted sea lion bones from Oregon.
I’m also pleasantly satisfied with additional press attention our Pelagiarctos
article got over the last couple of weeks, and the PLOS metrics indicates
it’s already gotten 3,000 views, which is quite a few more than my 2011 fur
seal article (under 100), although that’s probably because it’s 1) linked to in
all the news articles and 2) not paywalled.

Comparison of the holotype (B, C) and referred mandible (A) of Pelagiarctos. From Boessenecker and Churchill (2013).

When Morgan and I started our research on the new specimen
of Pelagiarctos, we realized that the most interesting application of
the research would be reevaluating the interesting hypothesis by Barnes (1988)
that Pelagiarctos thomasi was a specialized macrophagous predator. To
recap, Barnes interpreted this novel hypothesis based on several lines of
evidence: 1) Pelagiarctos thomasi is relatively rare in the Sharktooth
Hill Bonebed relative to other pinnipeds (apex predators are rare because they
require a large population of prey items to subsist upon), 2) sharp postcanine
teeth roughly similar in morphology to those of bone-cracking hyaenids and
borophagine canids, 3) a fused mandibular symphysis, suggesting an adaptation
towards high bite force, and 4) large body size. In addition to these
interpretations, Barnes (1988) further interpreted all the isolated teeth as
being from males – as the holotype “chin” has large canines, and at least one
of the postcanine teeth slipped right in to the empty alveolus of the holotype.
The rest of the postcanines are of similar size, suggesting to Barnes that they
were all from one gender. Most pinnipeds – and all fossil walruses for which we
have sufficient sample sizes (even wee little Proneotherium) are
sexually dimorphic, with larger males. Furthermore, one of the holotype canines
was broken in life and then worn from continued use, which Barnes (1988)
interpreted as the result of male-male combat. Barnes further speculated that
the lack of females could be caused by geographic separation of sexes –
certainly an intriguing possibility, but difficult to test with such a small
sample size.

An example of a vagrant pinniped: a leopard seal that wound up on a New Zealand beach. Apparently this happens somewhat often, but regardless - New Zealand is not in the normal range of this animal. From keaphotography.org

We identified several other hypotheses which
could just as parsimoniously explain the rarity of Pelagiarctos within
the Sharktooth Hill Bonebed. Given the extremely large sample size of fossil
vertebrates from the bonebed, and a century of intensive collecting, the rarity
is probably a real phenomenon and thus probably not a result of preservational
bias (least of which because Pelagiarctos is a large animal and has a
higher preservation potential than the smaller but numerically more abundant Neotherium).
Numerous studies of modern pinnipeds have demonstrated that they are prone to
vagrancy – in the ocean, after all, it’s easy to get caught up in currents or
forage further away than other members of your species. It’s also possible that
Pelagiarctos was simply a pelagic,
offshore pinniped, rarely straying into coastal waters off OrangeCounty (“Topanga”
Formation) or the TemblorSea (Sharktooth Hill). Furthermore, the Sharktooth Hill Bonebed was
deposited over a protracted period of time (~700,000 years) due to a
depositional hiatus, and it’s possible that a short period of time could have
seen introduction of Pelagiarctos (from further south, north, or further
out in the Pacific) along with a brief change in climate, ecology, or
circulation.

A sea lion and a fishy smorgasbord. Photo by David Doubilet.

The fused mandibular symphysis is a bit more ambiguous. The
only modern pinniped with a fused symphysis is the extant walrus Odobenus
rosmarus, and it’s not immediately clear why, or how it could be adaptive
relative to feeding. My hunch is that it’s a secondary consequence of having a
pachyosteosclerotic mandible – the lower jaw of Odobenus is thickened
around the chin and dense, and development of this may have resulted in
symphyseal fusion. The earlier walrus Alachtherium/Ontocetus has
an unthickened chin and lacks fusion – but Valenictus chulavistensis
(the strange sister taxon of Odobenus) has a fused symphysis, and also
lacks a thickened “chin”. Extreme pachyosteosclerosis of the skull and mandible
in Odobenus has been suggested as a possible adaptation for keeping the
head negatively buoyant during benthic foraging. One other fossil odobenid has
a fused symphysis – Dusignathus seftoni from the San Diego Formation.
It’s not even clear what Dusignathus ate, so it’s not a very good
analogue either. Barnes (1988) argued that a fused symphysis in Pelagiarctos
thomasi suggested and adaptation for large bite forces – however,
carnivorans with high bite forces such as borophagine canids and hyaenids
(which Barnes compared the dentition of Pelagiarctos to), as well as sea
otters – all have unfused symphyses, possibly to allow slight movement of the
mandibles so as to avoid tooth damage (Scapino, 1981). So what was mandibular
fusion in Pelagiarctos thomasi for? Who knows! That’s for someone else to
figure out. Besides, the new specimen didn’t have a fused symphysis anyway.

Comparison of calculated trophic level and body mass in pinnipeds. Lower trophic level corresponds to feeding upon benthic invertebrates, and high trophic level corresponds to eating large fish and cephalopods. From Boessenecker and Churchill (2013).

A walrus after a successful seal kill. These events have only been witnessed a few times by humans. Apparently, walruses will use their powerful suction normally reserved for mollusks to literally suck the meat right from the bones. From moblog.net.

Another gory shot of a walrus feeding on a poor seal. From Vlasman and Campbell, Diseases and Parasites of Mammals of the Eastern Arctic.

We reinvestigated the issue of body size as well. Morgan has
been working on a method to estimate the body mass of fossil pinnipeds, and his
preliminary results indicated that the length of the lower toothrow is the
single best predictor of body mass in a dataset of modern pinnipeds (fortunate
for us, since all we had was a mandible to work with). Morgan was able to
estimate the body mass of Pelagiarctos sp. at approximately 350 kg (~770
lbs), which is similar to some modern sea lions (male South American sea lions,
California sea lions). There are
much larger sea lions, however – the Steller’s sea lion (Eumetopias jubatus),
which weigh up to 1,150 kg (2,500 lbs). Steller’s sea lion is the fourth
largest pinniped (after the two elephant seals and the modern walrus), but is not
a macrophagous predator: it feeds predominantly on fish, although it will
occasionally prey upon juvenile pinnipeds. The modern walrus also occasionally
preys upon juvenile seals and marine birds – yet it is clearly adapted and
specialized for mollusk predation. All large bodied otariids predominantly eat
fish as well, and many adult male sea lions of other species as well will also
occasionally consume warm blooded prey when given the opportunity. Morgan
further investigated this by calculating the trophic level of modern pinnipeds
and plotting it relative to body mass. The result is that there is no apparent
trend between body mass and diet – with one exception: the largest pinnipeds
fed both at the high and low trophic levels (e.g. fish, cephalopods, as well as
benthic invertebrates – e.g. walruses and bearded seals). In other words, large
body mass doesn’t necessarily indicate anything specific about feeding ecology
or diet. A previous analysis by Peter Adam and Annalisa Berta (2002) only found
a very poor correlation between morphology and diet. That evidence of
adaptations for macrophagy is lacking within pinnipeds is highlighted by the
leopard seal: it does not use its postcanine teeth to feed upon penguins and
seals, and only uses them for filter feeding for krill the rest of the year (8
months or so out of the year). When it feeds upon large bodied prey, it nips
with its incisors and canines – which are not really any different from those
of fish-eating pinnipeds. In other words, the only modern pinniped which could
be argued to be a macrophagous apex predator only has dental specializations
for feeding upon krill.

Arguably the only macrophagous pinniped - the leopard seal spends most of the year eating krill, and doesn't use its delicate postcanine teeth for killing penguins and seals, and instead only for filter feeding. Figures from Hocking et al. (2013).

So, not a "killer" walrus after all, but still a pretty intimidating beast.

So what did Pelagiarctos feed on? Probably fish,
cephalopods, the normal menu for large bodied pinnipeds. It very well probably
did feed upon warm blooded prey – occasionally, anyway (again, like modern
pinnipeds). We’re not arguing that Pelagiarctos did NOT eat warm blooded
prey – rather, we’ve made the case that Pelagiarctos lacked any
adaptations which would lend themselves to macrophagy. One last point of
interest – within the Sharktooth Hill Bonebed, Pelagiarctos is not even
the largest pinniped; it’s dwarfed by Allodesmus, which Morgan estimated
at 1400 kg (~3,000 lbs)! That’s just enormous (and approximately ¾ the size of
modern northern elephant seals). Allodesmus, on the other hand, had
enormous orbits possibly indicating deep diving adaptations (to which its large
body size may have helped with as well), and a long snout with simple teeth – a
definite contrast to the short “bulldog” face and dentition of Pelagiarctos.

Number of visits

About the Coastal Paleontologist

I'm a paleontologist and adjunct faculty at College of Charleston in South Carolina, with research interests in Cenozoic marine vertebrates with an emphasis on marine mammals (whales, dolphins, pinnipeds, otters, sea cows, and others), but I willingly entertain brief distractions into the worlds of marine birds, sharks, and fish. My M.S. (2011, MSU-Bozeman) focused on marine vertebrate taphonomy whilst my Ph.D. (2015, U. Otago, NZ) focused on Oligocene baleen whales from New Zealand. Current research is concerned with fossil cetaceans from South Carolina including Oligocene eomysticetids, toothed mysticetes, and archaic dolphins.